class WallState(object):
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return self.name

HARD_WALL = WallState("Hard Wall")
EMPTY_WALL = WallState("Empty Wall")

del(WallState)

class Wall(object):
    def __init__(self, state=HARD_WALL):
        self.state = state

    def change_to(self, state):
        if state not in (EMPTY_WALL, HARD_WALL):
            print "%s is not a wall state." % (state,)
        self.state = state
